The present invention is related to improved capacitors comprising thin electrodes with improved physical properties. More specifically, the present invention is related to ceramic capacitors comprising thin nickel based electrodes with refractory metals incorporated therein.
Ceramic capacitors, and their manufacture, are well known in the art. In general, ceramic capacitors comprise alternating layers of conductive metal and ceramic. The manner in which such capacitors are formed typically involves formation of a green, unfired, ceramic layer with metal layers on either side thereof. The green ceramic is then fired at high temperature to form sintered ceramic layers between conductive layers of metal.
A problem well known to the artisan is the difficulty associated with firing a ceramic to a temperature sufficient to achieve optimal properties while, at the same time, avoiding degradation of the metallic conductive layer. Attempts to solve this problem are legion yet none are totally adequate.
Many artisans have attempted to lower the sintering temperature of the ceramic layer as set forth in U.S. Pat. No. 5,011,803 or to minimize oxidation of the conductor as set forth in U.S. Pat. No. 5,600,533. Still others have attempted to develop ceramic coating solutions that readily calcine in substantially nonoxidizing atmosphere as set forth in U.S. Pat. Nos. 4,959,295 and 4,925,771; 4,912,019; 4,908,296 and 4,613,560.
Other artisans have utilized high melting point conductors such as those containing platinum and palladium in alloy with silver, for example. The high cost of platinum and palladium is contrary to the continual demand to lower the cost of capacitors.
Nickel is the preferred conductor in ceramic capacitors due to the low cost and adequate conductive properties. Unfortunately, nickel has a melting point of about 1,455° C. and is highly susceptible to oxidizing at firing temperatures. Nickel oxide is highly undesirable due to unacceptably low conductivity. If nickel oxide is used the nickel must be reduced after firing of the ceramic to achieve adequate results. This requires an additional processing step which is undesirable. Typically, the ceramic is fired in neutral or reducing atmosphere to maintain the nickel in metallic, or unoxidized, form. Unfortunately, the surface energy of nickel is high at elevated temperature in a reducing, or neutral, atmosphere. Due to the high surface energy nickel desires to pull away from the ceramic during firing. The result is inconsistent layer thickness, as can be visualized under magnification, and poor electrical properties, as can be realized in routine testing. To avoid these losses thick layers of nickel such as larger than 1.5 μm are typically employed which is contrary to the desire to lower cost.
It has been a long felt desire to be able to utilize nickel electrodes without the problems associated with poor surface energy at firing temperature or the requirement that thick layers be used. Prior to the present invention this problem has been considered insurmountable.